1) Field of the Invention
This invention relates to a novel color-developing composition comprising a multivalent-metal-modified salicylic acid resin, a color-developing sheet for pressure-sensitive copying paper sheets, said color-developing sheet having a layer of the color-developing composition, and also an aqueous suspension of the color-developing composition. In addition to pressure-sensitive copying paper sheets, the color-developing composition is also usable in heat-sensitive recording paper sheets, copying ink compositions, color-developing agents for transfer-type copying paper sheets, and the like.
2) Description of the Related Art
Pressure-sensitive copying paper sheets are also called "carbonless copying paper sheets". They produce a color by mechanical or impactive pressure, for example, by writing strokes or typewriter impression, thereby allowing the making of a plurality of copies at the same time. Among such pressure-sensitive copying paper sheets, there are those called "transfer type copying paper sheets", those called "self-contained copying paper sheets", etc Their color-producing mechanisms are each based on a color-producing reaction between an electron-donating colorless dyestuff precursor and an electron-attracting color-developing agent.
In general, a pressure-sensitive copying paper sheet is formed of a sheet (CB-sheet), which is coated with microcapsules of a non-volatile organic solvent containing an electron-donating organic compound (pressure-sensitive dyestuff), and another sheet (CF-sheet), which is coated with an aqueous coating formulation containing an electron-attracting color-developing agent, with their coated sides maintained in a face-to-face contiguous relation. The microcapsules are ruptured by the above-described printing pressure, so that the pressure-sensitive dyestuff solution is caused to flow out into contact with the color-developing agent to develop a color. By changing the combination of a microcapsule layer, which contains a pressure-sensitive dyestuff, and a color-developing layer, it is possible to make a plurality of copies or to produce pressure-sensitive copying paper sheets capable of producing a color individually (SC-sheets).
Taking a pressure-sensitive copying paper of the transfer type by way of example, it will be described with reference to FIG. 1 which is a schematic cross-sectional view showing the structure of the illustrative pressure sensitive copying paper sheet.
The back sides of a CB-sheet 1 and CF/CB-sheet 2 are coated with microcapsules 4 which have diameters of several micrometers to somewhat greater than 10 micrometers and have been obtained by dissolving a colorless pressure-sensitive dyestuff precursor in a non-volatile oil and then encapsulating the resultant pressure-sensitive dyestuff precursor solution with high molecular films such as gelatin films. On the other hand, the front sides of the CF/CB-sheets 2 and a CF-sheet 3 are coated with .a coating formulation containing a color-developing agent 5 which has properties such that upon contact with the pressure-sensitive dyestuff precursor, the color-developing agent 5 undergoes a reaction with the dyestuff precursor, thereby causing the dyestuff precursor to produce its color. In order to make copies, they are stacked in the order of the CB-sheet, (CF/CB-sheet) and CF-sheet with the sides coated with the dyestuff precursor maintained in contiguous relation with the sides coated with the color-developing agent. When a pressure is applied locally by a ball-point pen 6 or a typewriter, the capsules 4 are ruptured there. As a result, the solution containing the pressure-sensitive dyestuff precursor is transferred to the color-developing agent 5 so that one or more copied records are obtained.
Illustrative colorless or light-colored dyestuff precursors usable in such pressure-sensitive copying paper sheets include:
Triarylmethanephthalide compounds such as Crystal Violet lactone. PA1 Fluoran compounds such as 3-dibutylamino-6-methyl-7-anilinofluoran. PA1 Pyridylphthalide compounds. PA1 Phenothiazine compounds. PA1 Leucoauramine compounds. PA1 a) polyvinyl alcohol derivatives containing at least one sulfonic acid group in the molecules thereof, and salts thereof; and PA1 b) polymers and copolymers containing as an essential component a styrenesulfonic acid salt represented by the following formula (IV): ##STR3## wherein R.sub.7 means a hydrogen atom or an alkyl group having 1-5 carbon atoms and M' denotes Na.sup.+, K.sup.+, Cs.sup.+, Fr.sup.+ or NH.sub.4.sup.+.
One of more dyestuff precursors selected from these dyestuff precursors are dissolved in a hydrophobic high-boiling-point solvent and microencapsulated for use in the production of pressure-sensitive copying paper sheets.
As electron-attracting color-developing agents, there have been proposed (1) inorganic solid acids such as acid clay and attapulgite, as disclosed in U.S. Pat. No. 2,712,507; (2) substituted phenols and diphenols, as disclosed in Japanese Patent Publication No. 9309/1965; (3) p-substituted phenol-formaldehyde polymers, as disclosed in Japanese patent Publication No. 20144/1967; and (4) metal salts of aromatic carboxylic acids, as disclosed in Japanese Patent Publication Nos. 10856/1974, 25174/1976, 1327/1977, etc. Some of them have already been employed actually.
Performance requirements which a color-developing sheet is supposed to satisfy include (1) high density of color marks produced at room temperature, (2) small density reduction of produced color marks during long-term storage, (3) high color-developing speed of color marks especially at low temperatures, (4) reduced yellowing of paper surface during storage or upon exposure to radiant rays such as sunlight, (5) high resistance of produced color marks to disappearance or fading upon contact with water or a plasticizer and (6) high resistance of produced color marks to fading upon exposure to radiant rays such as sunlight.
Color-developing agents, which have been proposed to date, and sheets coated with such conventional color-developing agents have both advantages and disadvantages as will be described next.
1. Inorganic solid acids:
For example, inorganic solid acids are inexpensive but adsorb gas and moisture in the air during storage. They hence result in yellowing of paper surfaces and reduced color-producing performance. Color marks produced using inorganic solid acids undergo substantial fading when exposed to radiant rays such as sunlight.
2. Substituted phenols:
Substituted phenols have insufficient color-producing ability and produce color marks have a low color density. At low temperatures, the color-developing speed is low.
3. p-Substituted phenol-formaldehyde polymers:
p-Phenylphenol-novolak resins, which are primarily employed as p-substituted phenol-formaldehyde polymers, are excellent in the density of produced color marks, the color-developing speed at low temperatures and the resistance to water or a plasticizer, but paper sheets coated with them undergo yellowing and produced color marks become therefor significantly faded upon exposure to radiant rays such as sunlight or during storage (especially, by nitrogen oxides in the air).
4. Metal salts of aromatic carboxylic acids:
As color-developing agents capable of improving the drawbacks of conventional color-developing agents, some metal salts of aromatic carboxylic acids, especially metal salts of salicylic acid derivatives have been proposed.
4-1. Mixtures containing a metal salt of 3,5-di(.alpha.-methylbenzyl)salicylic acid
These mixtures include, for example, (i) mixtures of metal salts of 5-[.alpha.-methyl-4'-(.alpha.-methylbenzyl)benzyl]salicylic acid and metal salts of 3,5-di(.alpha.-methylbenzyl)salicylic acid, said mixtures being disclosed in Japanese Patent Laid-Open Nos. 100493/1986, 084045/1987 and 096449/1987 and being available from salicylic acid and phenyl ethanols or styrenes; and (ii) mixtures of metal salts of 3-.alpha.-methylbenzyl-5-(1,3-diphenylbutyl)salicylic acid and metal salts of 3,5-di(.alpha.-methylbenzyl)salicylic acid, said mixtures being disclosed in Japanese Patent Laid-Open Nos. 91042/1990 and 91043/1990 and being available from salicylic acid, styrene and derivatives thereof.
A color-developing agent obtained from salicylic acid compounds in accordance with any one of these proposed preparation processes generally comprises a mixture of two components from which resin components other than the salicylic acid component have been extracted out. Mixtures of such salicylic acid derivatives contain a metal salt of 3,5-di(.alpha.-methylbenzyl)salicylic acid, said metal salt being disclosed in Japanese Patent Publication No. 25174/1976 referred to above and have been employed actually, and the other salicylic acid component is a compound either identical or close in molecular weight to the 3,5-di(.alpha.-methylbenzyl)salicylic acid.
When these color-developing agents are used in copying or recording paper sheets, the coated paper surfaces are imparted with improved yellowing resistance, but the low-temperature color-developing ability, water or plasticizer resistance, light fastness and the like, which have heretofore been considered to present problems, are still not considered to have been improved.
4-2. Mixtures of a salicylic acid compound and a resin compatible therewith
With a view toward improving light fastness or water or plasticizer resistance, (iii) Japanese Patent Publication No. 1195/1980 proposes to use a salicylic acid compound in admixture with a resin compatible therewith. Such a method is certainly effective for the improvement of waterproofness and light fastness but is still insufficient with respect to the color-developing speed at low temperatures, the density of color marks produced at low temperatures and long-term stability.
Effects of a salicylic acid compound as a color-developing agent are dependent on its substituent group or groups. Therefore, color-developing ability is generally low even when a mere metal salicylate is used in combination with a compatible resin. Introduction of at least one aromatic substituent group into the skeleton of salicylic acid is therefore an essential requirement for salicylic compounds to be used in accordance with such a method. It has been pointed out, however, that 3,5-di-(.alpha.-methylbenzyl)salicylic acid known as a compound containing an aromatic substituent introduced into the skeleton of salicyclic acid is expensive because it is prepared by a high-temperature and high-pressure reaction.
4-3. Metal-modified salicylic acid resins
In attempts to improve the low-temperature color-developing ability and the water or plasticizer resistance, some methods have been proposed in recent years to resinify salicylic acid and to use its metal-modified products.
Examples of such attempts include (iv) metal-modified polybenzylsalicylic acids obtained from salicylic acid and benzyl halides, as disclosed in Japanese Patent Laid-Open No. 132857/1988; (v) metal-modified salicylic acid resins obtained from salicylic acid and styrenes, as disclosed in Japanese Patent Laid-Open No. 112537/1988; and (vi) metal-modified salicylic resins formed from salicylic acids and various benzyl derivatives, as proposed by the present inventors in Japanese Patent Laid-Open No. 186729/1988, Japanese Patent Laid-Open No. 254124/1988, Japanese Patent Laid-Open No. 289017/1988, Japanese Patent Laid-Open No. 56724/1989 and Japanese Patent Laid-Open No. 77575/1989.
It is stated as an advantage that the low-temperature color-developing speed and waterproofness are generally improved to significant extents when these metal-modified salicylic acid resins are used as color-developing agents.
The above-exemplified, multivalent-metal-modified salicylic resins are not considered to contain any compatible, organic high-molecular weight compounds. It is, therefore, still difficult to say that such color-developing agents have sufficiently solved the problems in light fastness and long-term storage stability of produced color marks.
To produce a pressure-sensitive copying paper sheet by using a color-developing agent, the color-developing agent is generally wet-ground in the presence of a surfactant so that the color-developing agent is formed as fine particles having a particle size of 1-10 .mu.m into an aqueous suspension. Upon formation of the suspension, a dispersant is also used. The selection of a combination of particles to be dispersed and a dispersant for the provision of a good dispersion system practically relies upon experiences in many instances, and there is no general rule therefor. When a dispersant is chosen, it is necessary to take into account not only its dispersing ability but also its interaction with dispersed particles. For example, for phenol-formaldehyde condensation products which have been employed as color-developing agents in pressure-sensitive copying paper sheets, an anionic high molecular weight surfactant of the polycarboxylic acid type, specifically the sodium salt of maleic anhydride-diisobutylene copolymer is usually used as a dispersant. However, if this dispersant is used upon formation of the color-developing composition, which comprises the above-described multivalent-metal-modified salicylic acid resin, into an aqueous suspension, a complex is inconveniently formed between the multivalent metal and the carboxylic acid salt in a substantial reduction in the dispersing ability and dispersion stability, production of hardly defoamable foams, changes in the physical properties of the color-developing agent due to modifications of the multivalent-metal-modified salicylic acid resin as a dispersoid, etc. It is therefore impossible to obtain any practically usable aqueous suspension. Salts of naphthalenesulfonic acid-formaldehyde condensation products, salts of ligninsulfonic acid, and the like--which were previously employed for color-developing agents of the phenol-formaldehyde condensation products--include those capable of showing dispersing ability for color-developing compositions comprising a multivalent-metal-modified salicylic acid resin. When they are employed in pressure-sensitive copying paper sheets, the pressure-sensitive copying paper sheets are accompanied by drawbacks such as coloration, light yellowing or the like of the paper surfaces due to the dispersants themselves so that such dispersants substantially lack practical utility.
It is accordingly not easy to combine a color-developing composition, which comprises the above-described multivalent-metal-modified salicylic acid resin, with a suitable dispersant into an aqueous suspension having good quality in various properties such as dispersibility, stability and color-developing ability.